1. Field Of The Invention
The present invention relates generally to identifying the period of a waveform. More particularly, the present invention relates to a method and apparatus for identifying the period of an AC waveform to provide information, such as to an electrical to a relay, which may be used in monitoring or control functions, such as to provide protection from phase loss, ground fault, and overcurrent conditions.
2. Description Of The Related Art
Many applications require analysis of AC waveforms. For example, it is often desirable to know the exact root-mean-squared (RMS) content of a signal, such as AC power. This can be accomplished by means of a hardware or software calculation method. For software calculation algorithms to function correctly, a timing signal is usually needed to tell the software when to start calculating the RMS content of a sampled waveform, and when to stop. This timing signal may take the form of a square wave with edges corresponding to zero crossings of the sampled signal, and having the same frequency as the sampled signal. Thus, the zero crossings will indicate the period of a particular waveform being sampled.
In such applications, inaccuracies in timing signals may result in corresponding errors in any analysis done on waveforms sampled based upon the timing signals. For example, the timing signal may be out of phase with or have a different period from the sampled waveform. By calculating RMS values for a sampled waveform in which the period is inexact, the RMS values calculated by a system may be inaccurate. This becomes particularly significant when the RMS values are being used in a system for circuit protection or monitoring, such as provide protection from phase loss, ground fault, and overcurrent conditions. What is needed is a way of identifying an accurate period of a sampled waveform to be used in RMS calculations.
Circuits have been developed for creating timing signals, such as for RMS analysis of waveforms, based upon zero crossings of the waveform. Because between zero crossings of an alternating current waveform, the period may be assumed to be captured (from a first to a third zero crossing), zero crossings normally provide an adequate mechanism for identifying the waveform. However, depending upon the amplitude of the waveform, the point at which the zero crossing actually takes place may be difficult to discern. This is particularly true where large variations in amplitude may take place within specific circuitry. Where the amplitude is large, zero crossings may be clearly identified. However, where the amplitude becomes smaller, the waveform approaches the zero crossing from a more shallow angle, when considered graphically, making identification of the zero crossing difficult. If highly accurate analysis of the waveform parameters is needed, then, the timing signal based upon zero crossing may not be sufficiently accurate to ensure the desired degree of reliability.
There is a need, therefore, for an improved technique for identifying waveform periods. There is a particular need for a technique capable of reliably identifying waveform periods, or at least repeatable events in waveforms, so as to permit analysis to be performed based upon accurate timing signals through a range of amplitudes of the waveforms.
The present invention provides a technique for identifying a waveform period designed to respond to these needs. The technique may be used with a variety of waveform profiles, but is particularly well suited to AC waveforms, such as voltage or current waveforms of AC power. The technique may also be used in a wide variety of devices and settings. However, the technique is particularly well suited to monitoring and control equipment, such as circuit protection devices wherein RMS calculations are made based upon sampling of AC waveforms. Other settings might include power monitors, control devices, power generation devices, and so forth.
The technique permits identification of waveform period over a range of amplitudes by providing a mechanism for identifying an event in a waveform other than a zero crossing. In particular, the technique identifies points at which a waveform crosses a level offset from a zero axis. By detecting the event offset from the zero axis, the timing signals produced by the circuitry are more immune to noise which is particularly elevated in comparison to lower amplitudes which the waveform may assume.